Turbidimetry is a science of measuring decreased intensity of light caused by scattering or absorption of the light by an inhomogeneous system. The light scattering and absorption can be caused, for example, by an inhomogeneous system comprising solid particles distributed in a liquid, wherein the solid and the liquid have different indices of refraction, or by an inhomogeneous mixture of two liquids having different indices of refraction.
Turbidity is a measurable value for a sample. Turbidity of a sample can be related to an intensity of an incident light and an intensity of a transmitted light (after scattering and assuming there is no absorption of the light) by the following expression:I=I0e−τL 
wherein I0 is the intensity of the incident light, I is the intensity of the transmitted light, τ is turbidity, and L is an optical path length, i.e., a distance through the sample that the light traverses.
Measurements of turbidity are typically reported in units called Nephelometric Turbidity Units (NTU), also called nephelos turbidity units. When suspended formazin is tested in a nephelometer apparatus, the turbidity units are referred to as Formazin Nephelometric Units (FNU). An instrument for measuring water turbidity is called a turbidimeter. Some turbidimeters are called haze meters, which can be used to measure turbidity of gas, including atmospheric gas. Units of turbidity also can be expressed as European Brewery Convention (EBC) units of the International Organization for Standardization (ISO) or Formazin Turbidity Units (FTU) of the American Society of Brewing Chemists (also known as ASBC-FTU). Other units of turbidity are Formazin Attenuation Units (FAU).
Turbidimetry is used in a variety of applications. For quality control or public health purposes, turbidity, including haze, of chemical reactions, formulations, and quality testing samples is monitored using known turbidity assay methods. Turbidity assays measure a degree of “cloudiness,” haziness, or opaqueness of a test sample. Turbidity testing is performed with nephelometers, turbidimeters and haze meters in diverse industries such as personal care products, consumer products such as cleaning mixtures, and water and beverage quality testing. Turbidity of test sample aliquots is measured and referenced to turbidity calibration standards to assess progress and quality of process steps or quality testing samples.
Turbidity assay methods and instruments are known. In one type of turbidity assay method, for example, a nephelometer apparatus measures light from a light source beam that is scattered off suspended particles or a discontinuous liquid (e.g., suspended liquid droplets) at 90° from the light source beam. The light source beam typically is in near infrared wavelengths and are selected in order to reduce any potential effects of color, if any, in a test sample.
Robust single layer standards are required for calibrating conventional turbidity measuring instruments, including haze meters. A conventional single layer turbidity calibration standard consists of solid particles suspended in a liquid. An example of such a standard is AMCO Clear® (APS Analytical Standards, Inc., a subsidiary of GFS Chemicals, Inc., Powell, Ohio), which consists of styrene divinylbenzene sub-micrometer copolymer beads (121 nm average diameter) suspended in an ultra-pure aqueous media. The beads can be suspended in the aqueous media for a period of time due to a phenomenon known as Brownian motion, a random movement of small particles suspended in a liquid or gas medium caused by collisions of the particles with molecules of the medium.
Drawbacks of conventional turbidity standards include settling of suspensions and evaporation and yellowing of liquids upon exposure to turbidity measurement or storage conditions. In addition to these drawbacks, other weaknesses are known. For example, there are physical limitations of Brownian motion. Consequently, standards that rely on Brownian motion to maintain suspension of particles are limited by the size of particles that may be suspended in a liquid. Further, an AMCO standard designed for one turbidimeter cannot be reliably used with a different type of turbidimeter, even if these meters are from a same manufacturer. Also, formazin has been used as a standard, but dilutions of formazin are highly unstable. Further, while a “stabilized formazin” (e.g., STABLCAL™, Hach Company, Loveland, Colo.) is more stable than formazin, preparing stabilized formazin requires strictly following a special mixing protocol. And a refractive index of low level stabilized formazin standards is very different from that of low level formazin standards and from most ultra-pure turbidity water. Differences in refractive indices can lead to very different test results. So, a turbidimeter calibrated with stabilized formazin at low levels cannot be verified with formazin standards. Further, producing standards having stable distributed suspensions of a discontinuous liquid in a different, continuous liquid has been problematic.
Also, temperature-dependent measurements of turbidity such as in studies of properties of polymers (e.g., molecular weight distribution studies) require turbidity calibration standards to work across a wide temperature range from below 0° C. to above 150° C. For example, a polymer sample may be dissolved in a solution at or near a precipitating temperature and then the temperature of a resulting mixture is lowered so that the polymer begins to precipitate out of solution, thereby increasing turbidity of the mixture (see Cantow Manfred J. R., ed. Polymer Fractionation, 1967, Academic Press, pages 191 to 211). Under these conditions, water-based and other liquid-based turbidity calibration standards may freeze, resulting in a turbidity change; concentrate due to evaporation; degrade due to heating; expand volumetrically; or the like.
The above-mentioned drawbacks can preclude an use of a conventional turbidity standard or limit the standard's shelf life to as little as a few days before the standard has to be discarded or remixed, which may or may not restore a suspension to its previous state.
Also, where a test sample has two or more layers, for example an organic layer and an inorganic layer (e.g., an aqueous layer), robust multi-layer turbidity calibration standards that emulate the test sample over a range of layers, concentrations and temperatures are needed to calibrate turbidity measuring instruments. But only single layer turbidity calibration standards have been prepared and used. Turbidity of multilayer test samples conventionally is carried out by dividing the layers from each other, and then separately measuring the turbidity of each of the divided layers. In addition to having the drawbacks mentioned previously for single layer standards, multi-layer turbidity test samples tend to develop a rag layer at an interface between two layers. The rag layer is due to partial mixing of the layers, which then are unusable. Three or more layer standards would compound the rag layer stability problem intrinsic to multi-layer turbidity standards. Historically, multilayer turbidity standards comprised of two or more liquid-based layers were also expected to also develop rag layers, and thus are unknown.
As chemical reaction, formulation, and quality testing samples become more complex, greater numbers of single layer and multi-layer turbidity test samples are needed. Further, globalization of industry research and manufacturing is increasing a need to transfer turbidity calibration protocols from site to site. Stable and reliable single and multi-layer turbidity calibration standards are required for testing increasing numbers of turbidity test samples and for calibrating turbidity measuring instruments at one research site to different such instruments at another research site or manufacturing site.
There is an increasing need in diverse industries for stable single layer and multi-layer turbidity calibration standards to provide reliable reference points in analyses of turbidity of single and multi-layer reaction, formulation, and quality testing samples. The standards could be used for calibration and quality control purposes. Ideally, the standards would resist time- and temperature-dependent changes to turbidity (e.g., due to settling or agglomeration of suspended solids or discontinuous liquids), color (e.g., due to oxidation), composition (e.g., due to reaction), or concentration (due to evaporation or volume expansion upon heating), as well as resist migration of components from layer to layer.